1. Field of the Invention
The present invention relates to a semiconductor memory device and a method of manufacturing the same. More particularly, the present invention relates to a magnetic random access memory (MRAM) including a magnetic tunneling junction (MTJ) cell and a method of manufacturing the same.
2. Description of the Related Art
A magnetic random access memory (MRAM) is a memory device that writes and reads data using a phenomenon in which a resistance of a magnetic tunneling junction (MTJ) cell varies according to a magnetization direction of upper and lower magnetic films, which are separated by an insulating film in the MTJ cell.
FIG. 1 illustrates a cross-sectional view of a conventional MTJ cell included in a conventional MRAM.
Referring to FIG. 1, the conventional MTJ cell M1 of the conventional MRAM includes an amorphous buffering film 12 and a pinning film 14, which is a semi-ferromagnetic film, that are sequentially formed on a lower electrode 10. A pinned film 16 is formed on the pinning film 14. The pinned film 16 includes a first ferromagnetic film 16a, a metal film 16b, and a second ferromagnetic film 16c, which are sequentially formed on the entire surface of the pinning film 14. Arrows in FIG. 1 indicate directions of magnetic fields in the first and second ferromagnetic films 16a and 16c. It can be seen that a direction of a magnetic field in the first ferromagnetic film 16a is opposite to a direction of a magnetic field in the second ferromagnetic film 16c. 
Generally, a film including a metal film having magnetic films disposed on and under the metal film, e.g., the pinned film 16, is called a synthetic anti-ferromagnetic (SAF) film. In an SAF film, spin directions of upper and lower magnetic films are fixed in an antiparallel state due to a thickness of the metal film. In the SAF film, an exchange bias between the upper and lower magnetic films exceeds 1000 Oe. Thus, the thermal stability of the SAF film is superior to that of a single magnetic film. Accordingly, the SAF film has widely been used as a pinned film of an MTJ cell.
Referring again to FIG. 1, a tunneling film 22 is formed on the second ferromagnetic film 16c of the pinned film 16. A free magnetic film 24 and a capping film 26 are then sequentially formed on the tunneling film 22. The direction of the magnetic field of the free magnetic film 24 is determined by an external magnetic field. The capping film 26 protects the free magnetic film 24.
The MTJ cell generates a stray magnetic field, which is caused by the first ferromagnetic film 16a and/or the second ferromagnetic film 16c. This stray magnetic field affects the free magnetic film 24 by causing a kink when the free magnetic film 24 is switched.
More specifically, the stray magnetic field affects a portion, in particular, an outer portion, of the free magnetic film 24 while the free magnetic film 24 is being switched. As a result, the affected portion of the free magnetic film 24 operates abnormally in that it is later in switching than other portions thereof or enters a substantially different magnetization state.
The effect of the stray magnetic field depends on the thicknesses of the elements constituting the MTJ cell M1. As an example, FIGS. 2 through are graphs showing the influence of a stray magnetic field on the switching characteristic of the free magnetic film 24 depending on thicknesses of the elements of the MTJ cell M1.
FIG. 2 shows the switching characteristic of the free magnetic film 24 when the first ferromagnetic film 16a, which is a lower magnetic film of the pinned film 16, is thicker than the second ferromagnetic film 16c, which is an upper magnetic film (hereinafter, the first case).
In FIG. 2, reference characters C1 and C2 denote a first magnetization state indicating unit and a second magnetization state indicating unit (hereinafter, first and second indicating units), which indicate the magnetization states of the free magnetic film 24 and the first and second ferromagnetic films 16a and 16c in the first case. In the first and second indicating units C1 and C2, two lowermost arrows indicate the magnetization state of the first ferromagnetic film 16a, an intermediate arrow indicates the magnetization state of the second ferromagnetic film 16b, and an uppermost arrow indicates the magnetization state of the free magnetic film 24.
Referring to FIG. 2, in the first case, there exists a region P1 in which the free magnetic film 24 is abnormally switched in a switching characteristic curve of the free magnetic film 24. Thus, a switching magnetic field is shifted.
FIG. 3 shows a switching characteristic of the free magnetic film 24 when the second ferromagnetic film 16c is thicker than the first ferromagnetic film 16a (hereinafter, the second case).
In FIG. 3, reference characters C3 and C4 denote a third magnetization state indicating unit and a fourth magnetization state indicating unit (hereinafter, third and fourth indicating units), which indicate the magnetization states of the free magnetic film 24 and the first and second magnetization state indicating units 16a and 16c in the second case. In the third and fourth indicating units C3 and C4, a lowermost arrow indicates the magnetization state of the first ferromagnetic film 16a, two intermediate arrows indicate the magnetization state of the second ferromagnetic film 16c, and an uppermost arrow indicates the magnetization state of the free magnetic film 24.
Referring to FIG. 3, in the second case, there exists a region P4 in which the free magnetic film 24 is abnormally switched in a lower right portion of a switching characteristic curve of the free magnetic film 24. Thus, a switching magnetic field is shifted.
FIG. 4 shows a switching characteristic of the free magnetic film 24 when the first ferromagnetic film 16a is as thick as the second ferromagnetic film 16c and both are relatively thick (hereinafter, the third case).
In FIG. 4, reference characters C5 and C6 denote a fifth magnetization state indicating unit and a sixth magnetization state indicating unit (hereinafter, fifth and sixth indicating units), which indicate the magnetization states of the free magnetic film 24 and the first and second ferromagnetic films 16a and 16c in the third case. In the fifth and sixth indicating units C5 and C6, two lowermost arrows indicate the magnetization state of the first ferromagnetic film 16a, two intermediate arrows indicate the magnetization state of the second ferromagnetic film 16c, and an uppermost arrow indicates the magnetization state of the free magnetic film 24.
Referring to FIG. 4, in the third case, there exist regions P2 and P3 in which the free magnetic film 24 is abnormally switched in left and right portions of a switching characteristic curve of the free magnetic film 24.
Thus, a switching magnetic field is shifted.
FIG. 5 shows a switching characteristic of the free magnetic film 24 when the first ferromagnetic film 16a is as thick as the second ferromagnetic film 16c and both are relatively thin (hereinafter, the fourth case).
In FIG. 5, reference characters C7 and C8 denote a seventh magnetization state indicating unit and an eighth magnetization state indicating unit (hereinafter, seventh and eighth indicating units), which indicate the magnetization states of the free magnetic film 24 and the first and second ferromagnetic films 16a and 16c. In the seventh and eighth indicating units C7 and C8, a lowermost arrow indicates the magnetization state of the first ferromagnetic film 16a, an intermediate arrow indicates the magnetization sate of the second ferromagnetic film 16c, and an uppermost arrow indicates the magnetization state of the free magnetic film 24.
Referring to FIG. 5, in the fourth case, although there is no region in which the free magnetic film 24 is abnormally switched in a switching characteristic curve of the free magnetic film 24 as opposed to the first through third cases. However, a switching magnetic field is still shifted.